The invention relates to a method for operating a glow plug by means of pulse-width-modulated voltage pulses and also to a glow plug control device.
Modern glow plug control devices have a load transistor for each glow plug connection terminal, said transistor being switched by a control unit between its conductive state and its blocking state. Pulse-width-modulated voltage pulses are thus generated and applied to a glow plug. The duty cycle of the pulse-width-modulated voltage pulses is adapted by the glow plug control device in relation to the strength of the heating current flowing through the load transistor and the glow plug, either so as to regulate the glow plug temperature or in order to feed a predefined electric power into the glow plug so as to control the glow plug temperature.
To measure the current flowing through a load transistor, current measurement circuits are known which have a sense transistor or other semiconductor element connected in parallel to the load transistor. With current measurement circuits of this type, which are described for example in DE 10 2009 046 181 A1, the current flowing through the load transistor is proportional to the measurement current flowing through the sense transistor. This proportionality factor is referred to as the kILIS factor or k-factor, such that the value of the current flowing through the load transistor can be calculated by multiplying the measurement current by the kILIS factor.
Load transistors comprising such current measurement circuits are commercially obtainable as integrated semiconductor elements. For example, STMicroelectronics provides such a semiconductor element identified by the following number: VND5004A-E. The kILIS factor is specified on the manufacturer's datasheet.